Georges Durry

A warm layer in Venus' cryosphere and high-altitude measurements of HF, HCl, H2O and HDO

Venus has thick clouds of H2SO4 aerosol particles extending from altitudes of 40 to 60 km. The 60–100 km region (the mesosphere) is a transition region between the 4 day retrograde superrotation at the top of the thick clouds and the solar–antisolar circulation in the thermosphere (above 100 km), which has upwelling over the subsolar point and transport to the nightside. The mesosphere has a light haze of variable optical thickness, with CO, SO2, HCl, HF, H2O and HDO as the most important minor gaseous constituents, but the vertical distribution of the haze and molecules is poorly known because previous descent probes began their measurements at or below 60 km. Here we report the detection of an extensive layer of warm air at altitudes 90–120 km on the night side that we interpret as the result of adiabatic heating during air subsidence. Such a strong temperature inversion was not expected, because the night side of Venus was otherwise so cold that it was named the ‘cryosphere’ above 100 km. We also measured the mesospheric distributions of HF, HCl, H2O and HDO. HCl is less abundant than reported 40 years ago. HDO/H2O is enhanced by a factor of ∼2.5 with respect to the lower atmosphere, and there is a general depletion of H2O around 80–90 km for which we have no explanation.

Atmospheric CH 4 and H 2 O monitoring with near-infrared InGaAs laser diodes by the SDLA, a balloonborne spectrometer for tropospheric and stratospheric in situ measurements

The Spectromètre à Diodes Laser Accordables (SDLA), a balloonborne spectrometer devoted to the in situ measurement of CH(4) and H(2)O in the atmosphere that uses commercial distributed-feedback InGaAs laser diodes in combination with differential absorption spectroscopy, is described. Absorption spectra of CH(4) (in the 1.653-microm region) and H(2)O (in the 1.393-microm region) are simultaneously sampled at 1-s intervals by coupling with optical fibers of two near-infrared laser diodes to a Herriott multipass cell open to the atmosphere. Spectra of methane and water vapor in an altitude range of approximately 1 to approximately 31 km recorded during the recent balloon flights of the SDLA are presented. Mixing ratios with a precision error ranging from 5% to 10% are retrieved from the atmospheric spectra by a nonlinear least-squares fit to the spectral line shape in conjunction with in situ simultaneous pressure and temperature measurements.

Diode laser spectroscopy of CO2 in the 1.6 μm region for the in situ sensing of the middle atmosphere

Abstract A diode laser spectrometer was used in the laboratory to study CO2 line intensities and pressure-broadening coefficients near 1.6 μm . The spectral region ranging from 6230 to 6250 cm −1 which is suitable for the in situ sensing of carbon dioxide in the lower stratosphere was studied using a commercial telecommunication-type diode laser. Thirteen lines of the (3001)III←(000) band of CO2 have been studied. The results of intensity measurements are compared to previous determinations and available databases. Furthermore the broadening coefficients by N2 and O2 for the strongest transitions are also reported and analyzed. Finally, preliminary measurements of stratospheric CO2 achieved in 2002 with the “SDLA” balloonborne TDL-spectrometer are discussed.

Shot-noise-limited dual-beam detector for atmospheric trace-gas monitoring with near-infrared diode lasers

A dual-beam detector is used to measure atmospheric trace species by differential absorption spectroscopy with commercial near-infrared InGaAs laser diodes. It is implemented on the Spectromètre à Diodes Laser Accordables, a balloonborne tunable diode laser spectrometer devoted to the in situ monitoring of CH4 and H2O. The dual-beam detector is made of simple analogical subtractor circuits combined with InGaAs photodiodes. The detection strategy consists in taking the balanced analogical difference between the reference and the sample signals detected at the input and the output of an open optical multipass cell to apply the full dynamic range of the measurements (16 digits) to the weak molecular absorption information. The obtained sensitivity approaches the shot-noise limit. With a 56-m optical cell, the detection limit obtained when the spectra is recorded within 8 ms is approximately 10(-4) (expressed in absorbance units). The design and performances of both a simple subtractor and an upgraded feedback subtractor circuit are discussed with regard to atmospheric in situ CH4 absorption spectra measured in the 1.653-microm region. Mixing ratios are obtained from the absorption spectra by application of a nonlinear least-squares fit to the full molecular line shape in conjunction with in situ P and T measurements.

First direct simultaneous HCl and ClONO2 profile measurements in the Arctic Vortex

Coincident vertical profiles of HCl and ClONO2 in the Arctic vortex have been retrieved simultaneously in the altitude range 13 to 31km. Solar occultation spectra were recorded, during both ascent and occultation, with a balloon-borne infrared Fourier transform spectrometer launched on 22 March 1995 from Kiruna (Sweden, 67°N, 22°E). A depleted layer of HCl is observed between 16 and 18 km, whereas ClONO2 is showing values larger than 2 ppbv around 20 km. The impact on the derived total inorganic chlorine of a 20% increase of the current ClONO2 absolute line intensities is discussed. These first direct measurements of the mixing ratio profiles of HCl and ClONO2 are compared with the results of the REPROBUS chemistry transport model and discussed in term of chlorine partitioning.

Pressure broadening and shift coefficients of H2O due to perturbation by N2, O2, H2 and He in the 1.39 μm region: experiment and calculations

Near-infrared diode laser spectrometry was used to determine the pressure broadening and shift effect on H2O due to N2, O2, H2 and He in the 1.39 µm region. Halfwidths and shifts of water vapour were measured for six transitions. These lines are from the ν 1+ν 3 and 2ν 1 bands. A complete set of H2O transitions with various J values was investigated, including the lines selected to monitor in situ stratospheric H2O with the SDLA, a balloon-borne diode laser spectrometer. Experimental results are compared with theoretical calculations based on a semi-empiric technique that incorporates various corrections to the Anderson theory. This approach is performed in the framework of the impact approximation, which makes interpretation of the collision process simpler and allows reliable results to be obtained.

In situ measurements of H 2 O from a stratospheric balloon by diode laser direct-differential absorption spectroscopy at 1.39 µm

A distributed-feedback InGaAs laser diode emitting near 1.393 µm is used in conjunction with an optical multipass cell that is open to the atmosphere to yield ambient water-vapor measurements by infrared absorption spectroscopy. To obtain the high dynamic range for the measurements that is required for continuous water-vapor monitoring in the upper troposphere and the lower stratosphere, we used a simple circuit that combined differential and direct detection. Furthermore, the laser emission wavelength was tuned to balance the steep decrease in H2O concentration with altitude by sweeping molecular transitions of stronger line strengths. The technique was implemented by use of the Spectrometre a Diodes Laser Accordables (SDLA), a tunable diode laser spectrometer operated from a stratospheric balloon. Absorption spectra of H2O in the 5–30-km altitude range obtained at 1-s intervals during recent balloon flights are reported. Water-vapor mixing ratios were retrieved from the absorption spectra by a fit to the full molecular line shape in conjunction with in situ pressure and temperature measurements, with a precision error ranging from 5% to 10%.

Diode laser spectroscopy of H2O in the 7165– range for atmospheric applications

Abstract A near-infrared diode laser spectrometer was used in the laboratory to measure H2O line intensities near 1.39 μm . The spectral region ranging from 7165 to 7186 cm −1 which is of interest for the in situ monitoring of H2O in the middle atmosphere from balloon or airborne platforms was studied. A temperature-stabilized White cell was used to perform measurements between room temperature and −60°C. A great care was taken in preventing corruption of the H2O measurements by ambient water vapor by using optical fibers to conduct light and by installing the complete spectrometer in a closed box filled with dry nitrogen at atmospheric pressure. Twenty-three transitions of ν1+ν3 and 2ν1 bands have been studied. The results are carefully compared to previous determinations and available databases. The main discrepancies are discussed. Finally, in situ stratospheric H2O spectra obtained recently from the SDLA, a balloonborne diode laser spectrometer, that were processed by means of the achieved H2O parameters are reported.

Balloon-borne near-infrared diode laser spectroscopy for in situ measurements of atmospheric CH4 and H2O

Absorption spectroscopy with near-infrared telecommunication laser diodes is a very convenient technique to measure in situ methane and water vapor in both the troposphere and the lower stratosphere (LS) and thereby to address many topics in the science of the atmosphere. This technique offers a high temporal resolution that ranges from 10 ms to 1 s, a precision error in the concentration retrieval of within a few percents and a dynamic range for the measurements of four orders of magnitude. A balloon-borne near-infrared diode laser spectrometer is described that provides simultaneous in situ methane (in the 1.65-microm region) and water vapor (in the 1.39-microm region) measurements at 1 s intervals. Tropospheric and stratospheric vertical concentration profiles of methane and water vapor are reported.

In situ Measurement of H2O and CH4 with Telecommunication Laser Diodes in the Lower Stratosphere: Dehydration and Indication of a Tropical Air Intrusion at Mid-Latitudes

Telecommunication laser diodes emitting near 1.39 μ m and 1.65 μ m in combination with direct-differential absorption spectroscopy are efficient tools to monitor in situ stratospheric H2O andCH4 with a good precision error (a few percents), a high temporal resolution (ranging from 10 ms to 1 s), a large dynamic range in the concentration measurements (four orders of magnitude) and a high selectivity in the analyte species. To illustrate the capability of laser probing technique, we report balloonborne H2Oand CH4 simultaneous measurements obtained on October 2001 atmidlatitudes (43° N). The H2O vertical profile achieved with the lasersensor in the lower stratosphere is compared with the H2O data yielded by a balloonborne frost-point hygrometer. The total hydrogen mixing ratio in the lower stratosphere, 2[CH4] + [H2O], appears to beconstant at 7.5 ± 0.1 ppmv. Nevertheless, an unexpected largedehydration of ∼0.5 ppmv was detected by both the laser sensor and thehygrometer between 16 km and 23 km. We suspect the occurrence of a tropicalair intrusion into mid-latitudes. We support this interpretation using a high-resolution advection model for potential vorticity.